A hot runner injection molding apparatus typically includes nozzles that are heated to maintain melt therein at a controlled temperature. The nozzles are typically in contact with a mold component that defines one or more mold cavities. The mold cavities in the mold component are filled with melt that first passes through the nozzles. The mold component is then typically cooled in order to solidify the melt in the mold cavities, thus forming a plurality of molded parts, which are then ejected from the mold cavities.
Because the nozzles are typically heated, and the mold component is cooled for at least a portion of an injection molding cycle, it is desirable to have a relatively low heat transfer from the nozzles into the mold component. Many nozzle constructions have been proposed in the past to address this issue. Examples of such a nozzle construction is shown in U.S. Pat. Nos. 5,492,467 and 5,554,395, both to Hume et al.
The '467 patent teaches a hot runner nozzle having a nozzle body, an insert which is positioned in the nozzle body, and a bushing that surrounds the insert. The bushing contacts the mold component, to form a seal therebetween. The bushing and the insert are spaced apart so that a gap exists between them. During the injection process, molten material fills the gap between the insert and the bushing. With this configuration, some heat is lost from the insert, through the molten material, through the bushing and finally into the mold component. Furthermore, the molten material itself can make the disassembly of the nozzle difficult and time consuming.
The '395 patent teaches a multi-piece nozzle tip assembly including a tip piece, a tip surrounding piece and a mold component contact piece. The tip surrounding piece surrounds the tip piece and retains the tip piece in the nozzle. The tip surrounding piece is spaced from the tip piece. A mold component contact piece is provided between the tip piece and the mold component, to inhibit melt from filling the space between the tip piece and the tip surrounding piece. However, heat can be lost from the tip piece through the seal and into the mold component. In particular, the heat losses occur near the downstream end of the insert, where control over the temperature of the melt is particularly important.
Thus, there is a continuing need for new nozzle constructions that have further improved heat transfer efficiency.